The present invention relates to a process for preparing polyamides from a mixture of at least one lactam and water under polyamide-forming conditions wherein at least one adiabatic expansion is effected during the polymerization.
Polyamide, widely prepared by polymerization of caprolactam, has an equilibrium content, depending on the temperature, of from 8 to 15% of caprolactam monomer and caprolactam oligomers. These cause undesirable effects in further processing of the polymer product and are therefore usually removed by extraction with water, caprolactam-comprising water or alcohol or treatment with inert gases or vacuum treatment after pelletization.
The extraction with water typically gives rise to a wash water extract comprising from about 2 to 15% by weight of caprolactam monomer and caprolactom oligomers, which is processed for economic and environmental protection reasons to recover the caprolactam monomer and oligomer for recycling into the polymerization.
Accordingly, DD-A-213 936, DE-A-43 21 683 and U.S. Pat. No. 4,049,638, for example, describe processes for polycaprolactam preparation which permit the use in the polymerization of caprolactam having a water content of up to 15%. EP-A-745 631 discloses the reuse of aqueous extract solutions through addition of small amounts of a di- or polycarboxylic acid, since the extract will polymerize slower than caprolactam otherwise.
Since the extract further comprises appreciable proportions of cyclic oligomers which remain unchanged by the polymerization, various processes have been proposed for cracking these oligomers or converting them into linear oligomers. The oligomers are typically cracked with phosphoric acid or by means of high temperatures. For instance, U.S. Pat. No. 5,077,381 describes a process for cracking the oligomers at from 220 to 290xc2x0 C., preferably under superatmospheric pressure. The use of acetic acid and orthophosphoric acid for oligomer cracking is described in DD-A-213 936, for example.
Prior to recycling into the polymerization, the typically about 10% strength by weight extract first has to be worked up, i.e., typically by concentrating it. The workup normally takes the form of removing the water by distillation. DE-A-25 01 348 describes concentrating to more than 70% by weight in the absence of atmospheric oxygen by addition of fresh caprolactam to the wash water extract prior to the concentrating. EP-A-123 881 discloses the addition of caprolactam to the extract prior to the concentrating to prevent any precipitation of oligomers.
The concentrating is typically carried out in two stages. In the first stage, the 10% strength solution is concentrated to about 75-80% of caprolactam and oligomers using conventional evaporating means. After further evaporation of the solution to  greater than 98%, the solution is polymerized at once. The aqueous solution of caprolactam and caprolactam oligomers is not stable in storage at concentrations above 70-80%, since the oligomers in particular precipitate.
When this method of concentrating is applied to wash water extract from pigmented polyamides, problems arise, however. The polyamide comprises delustering TiO2 pigments added before the pelletization, preferably during the polymerization. These delustering TiO2 pigments are additionally surface-treated or coated with inorganic additives to improve UV light stability and to impart certain properties, such as dispersibility and particle size. On extraction of the polyamide chips, these inorganic assistants are partly extracted with the caprolactam and caprolactam oligomers and then separate out during the concentrating of the wash water extract, especially on the surfaces of the heat exchanger. This appreciably shortens the service life of the evaporation equipment and additionally gives rise to the possibility of blocking the heat exchanger. Similar problems with a coating of heat exchanger surfaces arise during the evaporation of water in the polymerization stage when the concentrate is recycled back into the caprolactam polymerization, since the concentrate likewise still includes these inorganics.
By way of solution to these problems, EP 306 872 proposes using pigments comprising less than 0.1% by weight of 100xc2x0 C. water solubles. However, this restriction to the choice of pigments is disadvantageous.
It is an object of the present invention to provide an improved process for working up and further processing the wash water extract of polyamides, especially pigmented polyamides, to produce, in a simple manner and without the aforementioned problems, a concentrated extract which can be recycled into the polymerization.
We have found that, surprisingly, this object is achieved when the wash water extract is concentrated to an extractables content of not more than 85% by weight and this solution is mixed with fresh lactam in a ratio of from 1:1 to 1:8. The resulting mixture then comprises from 0.5 to 13% by weight of water. It can be recycled into the polymerization without the aforementioned problems when the water is removed by adiabatic expansion during the polymerization.
The present invention accordingly provides a process for preparing polyamides from at least one lactam, especially caprolactam, with or without further monomers and customary additives and fillers, which comprises
a) concentrating the wash water extract from the extraction of polyamide to an extractables content of not more than 85% by weight,
b) adjusting the water content of the resulting concentrate to from 0.5 to 13% by weight by addition of fresh lactam,
c) subjecting the resulting mixture to a polymerization under polyamide-forming conditions, and
d) effecting at least one adiabatic expansion during the polymerization to reduce the water content.